Semiconductor processing tools often use components called “showerheads” to distribute process gases, e.g., reactants, across a semiconductor substrate during processing. Showerheads are often sized to be larger than the substrates with which they are designed to be used, and may also serve as an RF electrode for the purposes of capacitively-coupled plasma generation. A typical showerhead may include a faceplate that forms one wall of a plenum; the faceplate may generally be oriented to be parallel to, and facing, a substrate being processed using the showerhead. The faceplate may have a large number of through-holes distributed across it, and gases introduced into the plenum may flow through the through-holes and towards substrates being processed using the showerhead.